# Primary coil as autotransformer?

Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-uswest-dot-net>" <pkomen-at-zianet-dot-com>

I have a 12KV 60ma NST driving my coil.

It has a flat primary of 1/4 copper tubing spaced at 1/4 inch.  From the
start across is 10 1/4 inches, measured at 90 degrees from that it is 10 3/4
inches.  27 turns (probably excessive but I had the space and the slots)
tapped at 21.75.  Note that I had some problems with arcing from the end of
the outside turn to the tubing brought out from the center below the
windings (more than an inch of clearance).  I suspect that the
autotransformer effect was causing the breakdown.  The form I wound this on
is Plexiglas.  I wound the copper tubing working on a card table.  When I
slid the finished primary off the table, I got zapped with a large static
discharge.  For a week or more, every time I touched the primary tubing I
got a shock.

My question is this:  Is the induced voltage on the outside turns (past the
tap) proportional to the average voltage per turn of the center tapped
portion?  Do the center turns have an equal voltage per turn or is that
proportional to the inductance per turn?  Would the inductance ratio be the
same as the voltage ratio for the center driven turns versus the outside
floating turns?  If an experiment was run using 60 Hz low voltage, would it
compare to Tesla voltage and frequency?

I think I am better at posing questions than answering them.

Now that I think about it, here's a test setup.  Wall outlet to variac,
variac to power transformer (11v at 50 amps output), output of transformer
to primary coil tapped as is.  Dial up the variac to produce 5 volts output
to the primary coil (reactance of .1 ohm at 60 Hz), giving 50 amps into
primary.  Use voltmeter to measure across outside turns.  I'd monitor
current into the transformer using 5 amp AC meter to make sure that the
input current stays below about 4.6 amps or so.  Comments?  I'll try it and
let you know the results.

Word didn't like my spelling, so this didn't go yet.  Results:

Tapped at 20.75 turns with 5 Volt input to the primary (60 Hz), input amps
to transformer are 3.33.
Inner 5 turns	0.5 v
Next 5		0.9 v
Next 5		1.1 v
Next 5		1.2 v
17-22 1.2 v
18-23 1.1 v
21-26 0.6 v

Tapped at 21.75 turns with 5 volt input,  3.28 amps into Xfmr.
Inner 5		0.43 v
Next 5		0.8 v
Next 5		1.05 v
Next 5		1.15 v
16.75-21.75 1.175 v
21.75-26.75 0.6 v
22-27		just less than 0.6 v

The copper tubing was warm when I finished.  Readings were done on a Radio
Shack multimeter with 0.05 volt per division on the 5 volt AC scale. (1.175
was 3 1/2 divisions past 1 volt.)

Conclusions:
1. The voltage does vary from inner turns to outer.
2. The outer floating turns have voltage induced but are not coupled well
enough to match the outer driven turns.
3. Judging from the input current to the transformer, the impedance of the
primary coil is larger than calculated or other resistance is adding in.
One hundredth ohm would change the current by nearly 10%.